A surround sound processor operates to enhance a two-channel stereophonic source signal so as to drive a multiplicity of loudspeakers arranged to surround the listener, in a manner to provide a high-definition soundfield directly comparable to discrete multitrack sources in perceived performance. An illusion of space may thus be created enabling the listener to experience the fullness, directional quality and aural dimension or "spaciousness" of the original sound environment. The foregoing so-called periphonic reproduction of sound can be distinguished from the operation of conventional soundfield processors which rely on digitally generated time delay of audio signals to simulate reverberation or "ambience" associated with live sound events. These conventional systems do not directionally localize sounds based on information from the original performance space and the resulting reverberation characteristics are noticeable artificial.
To accomplish this end, a surround sound processor typically comprises an input signal conditioning and matrix circuit, a control voltage generator and a variable matrix circuit.
The input conditioning and matrix circuit usually provides for balance and level control of the input signals, generates normal and inverted polarity versions of the input signals, plus sum and difference signals, and in some cases generates phase-shifted versions thereof, and/or filters the signals into multiple frequency ranges as needed by the remainder of the processing requirements.
The control voltage generator typically includes a band-pass filter circuit, a directional detector circuit, and a servologic circuit. The band-pass filter circuit shapes the frequency response of the signals applied to the directional detector circuit so that this circuit responds similarly to the human ear. The directional detector measures the correlations between the signals which represent sounds encoded at different directions in the stereophonic sound stage, generating voltages corresponding to the directional location of the predominant sound. The servologic circuit uses these signals to develop control voltages for varying the gain of voltage controlled amplifiers in the variable matrix circuit in accordance with the sound direction and the direction in which it is intended to reproduce the sound in the surrounding loudspeakers.
The variable matrix circuit includes a number of voltage-controlled amplifiers and a separation matrix. The voltage-controlled amplifiers amplify the input matrix audio signals with variable gain, for application to the separation matrix, where they are used to selectively cancel crosstalk into different loudspeaker feed signals. The separation matrix combines the outputs of the input matrix and of the voltage-controlled amplifiers in several different ways, each resulting in a loudspeaker feed signal, for a loudspeaker to be positioned in one of several different locations surrounding the listener. In each of these signals, certain signal components may be dynamically eliminated by the action of the detector, control voltage generator, voltage-controlled amplifiers (VCA's) and separation matrix.
In U.S. Pat. No. 5,172,415 entitled "Surround Processor", Fosgate discloses a servologic control voltage generator which employs a width-modulated pulse train to vary the time constants applied to the control signals, in accordance with the difference between the raw detector output signals and the smoothed control signals resulting after they have been passed through the variable time-constant filters, thereby placing the modulating elements within a feedback loop, as shown in FIGS. 5-7 of that application and FIGS. 3, 4, 6 and 9b herein.
In U.S. Pat. No. 4,932,059, Fosgate discloses use of variable time constants operated by means of a width-modulated pulse train, the duty cycle of which is controlled by means of a signal level detector, and further by means of a one-shot responsive to signal "attacks". This one-shot is so designed that the output pulse duration is sufficiently long to ensure that the control signals reach their appropriate values fairly quickly, but is sufficiently short that very low transient intermodulation distortion occurs, such that the listener is unable to hear any artifacts in the decoding process. This scheme is described with reference to FIGS. 2-5, 6a and 6b of the above-referenced patent, and FIG. 5b of co-pending application Ser. No. 07/789,529.
A time constant processing circuit is provided for smoothing directional information signals produced by a detector circuit with continuously variable time constants in order to generate one or more control voltage signals. The circuit is responsive to both the amplitude and the rate of change of the directional information signals, such that as the difference between each of the directional information signals and its corresponding control voltage signal increases, the value of the corresponding time constant decreases, so as to allow the control voltage signal to more closely follow the directional information signal.
Furthermore, if the difference between these signals increases beyond a certain threshold value, in co-pending application Ser. No. 07/789,529, the one-shot is triggered, and causes the time constants applied to all of the directional information signals to be reduced to the minimum value for a short, predetermined period of time, so that the control voltage signals rapidly catch up with the directional information signals from the directional detector circuit When the difference between the direction information signals and their corresponding control voltage signals decreases, the corresponding time constant increases so as to provide very smooth processing of the audio information.
Typically such large and fast changes in the direction information signals correspond with sudden attacks in the information presented to the surround processor. While the use of such a one-shot is not essential, it has been shown to be more effective than relying only upon the normal action of the servo-logic processor itself, without this addition.
In the present invention, this one-shot is replaced by a new circuit which lengthens the duration of a detected signal direction change and which acts on a single pulse width modulator (PWM) circuit to reduce the time constants in both of a pair of variable time constant circuits responding to front-back and left-right directional information signals. This provides a more effective circuit with fewer components and lower cost, while improving the perceived performance of the surround sound processor.
In surround sound processors, much of the subtleties of the presentation are due to the characteristics of the direction detector and servologic circuit of the control voltage generator and of the VCA's. As these are further refined, the apparent performance becomes more transparent and effortless-sounding to the listener.